When the new trailer for Harry Potter and the Half-Blood Prince was released, none of the actual movie actors could attend because they were in London filming the movie. The footage in the trailer leaves very much to speculate. Everyone that keeps up with the Potter books knows there will be a scene featuring an attack on the Weasley's home, The Burrow, but the trailer showed us what the actual footage looks like. The attack shown in the trailer involves fireballs, and some weird versions of Demons flying throughout the house, that do not resemble any of the attacks that are mentioned in the book.
Dumbledore has a speaking part in the movie that is no-where to be found in the books. "Voldermort has chosen Draco Malfor for a mission." Also, in the book, Harry Potter spends most of his time convinced that Draco is up to something, while everyone else thinks that he is just blaming Malfor for everything. If Dumbledore says that Malfoy is working for Voldermort, well, that will change the entire direction of the movie from the book.
The new trailer (shown below), shows things we have all read from the books, but it looks better than you had probably imagined.
The Producer Alan Horn asked at the release of the trailer, if anyone has forgiven him for delaying the production, hell no we haven't, and releasing a trailer is just a tease!